Chromosomal evolution in Rodentia

Rodentia is the most species-rich mammalian order and includes several important laboratory model species. The amount of new information on karyotypic and phylogenetic relations within and among rodent taxa is rapidly increasing, but a synthesis of these data is currently lacking. Here, we have integrated information drawn from conventional banding studies, recent comparative painting investigations and molecular phylogenetic reconstructions of different rodent taxa. This permitted a revision of several ancestral karyotypic reconstructions, and a more accurate depiction of rodent chromosomal evolution.     

From Diploids to Allopolyploids: The Emergence of Efficient Pairing Control Genes in Plants

Polyploidy has played a major role in the evolution of higher plants. Precise control of chromosome pairing is vital for conferring meiotic regularity, and hence reproductive stability in allopolyploids. In this review, we examine whether strong evidence has accumulated for the presence and activity of pairing control genes in different allopolyploid species that are entirely bivalent forming and that display a strict disomic inheritance. We show that very good evidence has been adduced in Triticum species, Avena sativa, Festuca arundinacea, Brassica napus, Gossypium hirsutum, and G. barbadense, and in amphidiploids related to the diploid species Lolium perenne, L. multiflorum, and L. rigidum. More circumstantial evidence has been obtained for polyploids in the genera Aegilops, Hordeum, Nicotiana, and Coffea, which have received far less attention than the other species. Although these pairing regulators seem to control different processes operating throughout the premeiotic interphase and the meiotic prophase, little is known about their precise mode of action. We present three hypotheses that have been proposed to explain the origin and evolution of pairing control genes; none of them has been supported by direct evidence, and the origin of most pairing suppressors is still unknown. Accordingly, the study of pairing control genes is still an important task for understanding the stabilization and establishment of allopolyploid species.     

Chromosomal diversity in the genus Arvicanthis (Rodentia, Muridae) from East Africa: a taxonomic and phylogenetic evaluation

In this paper we discuss the contribution of cytogenetics to the systematics of Arvicanthis in East Africa, by reviewing all the known chromosomal cytotypes of the genus in the area. We also provide G‐ and C‐banding comparisons for two recently described karyotypes, provisionally named ANI‐5 (2n = 56, NFa = 62) and ANI‐6 (2n = 60, NFa = 72). This, therefore, brings the total number of known cytotypes in this area to 10. Five of these correspond to the species recognized by the latest rodent checklist, i.e. A. nairobae (2n = 62, NFa = 78), A. neumanni (2n = 52–53, NFa = 62), A. blicki (2n = 48, NFa = 62), A. abyssinicus (2n = 62, NFa = 64) and A. niloticus (2n = 62, NFa = 60–62). The taxonomic status of the remaining five cytotypes (A. cf. somalicus, 2n = 62 NFa = 62–63; ANI‐5, 2n = 56, NFa = 62; ANI‐6/6a 2n = 60, NFa = 72/76; ANI‐7, 2n = 56, NFa = 78; and ANI‐8, 2n = 44, NF = 72) is discussed. Finally, we reconstruct the phylogenetic relationships among all the known karyotypes on the basis of banding data available for the genus in Africa and show the occurrence of two main clades, each characterized by different types of chromosomal rearrangements. The times of the cladogenetic events, inferred by a molecular clock, indicate that karyotype evolution has accomplished almost all the dichotomic events from the end of the Miocene to the present day. The discovery of a large chromosomal differentiation between populations showing low genetic distances and intrapopulation chromosomal polymorphism suggests that the process of chromosomal differentiation in Arvicanthis is still ongoing and may possibly be responsible for speciation.     

Comparative cytogenetics and heterochromatic patterns in two species of the genus Acanthostracion (Ostraciidae: Tetraodontiformes)

Some groups of fish, such as those belonging to the Order Tetraodontiformes, may differ significantly in the amount and location of heterochromatin in the chromosomes. There is a marked variation in DNA content of more than seven-fold among the families of this Order. However, the karyoevolutionary mechanisms responsible for this variation are essentially unknown. The largest genomic contents are present in species of the family Ostraciidae (2.20–2.60 pg). The present study cytogenetically characterized two species of the family Ostraciidae, Acanthostracion polygonius and A. quadricornis, using conventional staining, C-bandings, Ag-NOR, CMA₃/DAPI, AluI, PstI, EcoRI, TaqI and HinfI restriction enzymes (REs) and double FISH with 18S and 5S rDNA probes. The karyotypes of both species showed 2n = 52 acrocentric chromosomes (FN = 52; chromosome arms) and pronounced conserved structural characteristics. A significant heterochromatic content was observed equilocally distributed in pericentromeric position in all the chromosome pairs. This condition is unusual in relation to the karyotypes of other families of Tetraodontiformes and probability is the cause of the higher DNA content in Ostraciidae. Given the role played by repetitive sequences in the genomic reorganization of this Order, it is suggested that the conspicuous heterochromatic blocks, present in the same chromosomal position and with apparently similar composition, may have arisen or undergo evolutionary changes in concert providing clues about the chromosomal mechanisms which led to extensive variation in genomic content of different Tetraodontiformes families.     

Cytogenetic analysis of three sea catfish species (Teleostei, Siluriformes, Ariidae) with the first report of Ag-NOR in this fish family

Despite their ecological and economical importance, fishes of the family Ariidae are still genetically and cytogenetically poorly studied. Among the 133 known species of ariids, only eight have been karyotyped. Cytogenetic analyses performed on Genidens barbus and Sciades herzbergii revealed that both species have 2n = 56 chromosomes and Cathorops aff. mapale has 2n = 52 chromosomes: Genidens barbus has 10 Metacentrics (M), 14 Submetacentrics (SM), 26 Subtelocentrics (ST), and 6 Acrocentrics (A), Sciades herzbergii has 14M, 20SM, 18ST and 4A, whereas Cathorops aff. mapale has 14M, 20SM, and 18ST. The nucleolus organizer regions (NORs) were found in a single chromosome pair on the short arm of a large-sized ST pair in Genidens barbus and on the short arm of a middle-size SM pair in Cathorops aff. mapale. Multiple NORs on the short arms of two large-sized ST pairs were found in Sciades herzbergii. The occurrence of diploid numbers ranging from 2n = 52 through 56 chromosomes and the presence of different karyotypic compositions, besides the number and position of NORs suggest that several numeric and structural chromosome rearrangements were fixed during the evolutionary history of this fish family.     

印度蔊菜与无瓣蔊菜形态变异特征的比较及分类关系

印度蔊菜(Rorippa indica)与无瓣蔊菜(R. dubia)的分类关系仍存在较大争议, 为阐明二者的分类学关系, 本研究综合利用形态指标测量、DNA相对含量检测、体细胞染色体观察和基于SSR分子标记的遗传距离分析等方法, 系统地比较了二者的分类学特征和细胞遗传学差异。结果表明: 印度蔊菜(2n = 48)为六倍体, 无瓣蔊菜(2n = 32)为四倍体。同时, 前者DNA相对含量约为150, 是后者的1.5倍。通过45对SSR分子标记的遗传距离分析得出, 相对于球果蔊菜(R. globosa)与欧亚蔊菜(R. sylvestris), 二者亲缘关系较近, 独立聚类为一支, 但在遗传距离为0.23处可以明确划分为两个种。同时, 利用角果长度和结籽密度两个形态指标可以将二者明显区分为两个种。另外, 二者存在明显的生殖障碍, 通过正反交授粉实验得出: 印度蔊菜与无瓣蔊菜自花授粉的结实率分别为97.73%和95.65%, 而以印度蔊菜为母本的杂交处理结实率为0, 以无瓣蔊菜为母本的杂交处理结实率为47.06%, 但其种子萌发率为0。综上所述, 印度蔊菜与无瓣蔊菜为两个种。     

Molecular Views of Human Origins

Over the last half century, comparative genomics has increasingly contributed to the definition, resolution and interpretation of human evolution. Early comparisons demonstrated that African apes and humans were more closely related and diverged later than commonly thought. However, it was difficult to determine the branching between humans, chimpanzees and gorillas. By the 1990s, sufficient biomolecular data had accumulated to demonstrate that chimpanzees and humans shared a common ancestor after the divergence of the gorilla. Current reconstructions place the divergence of humans and chimpanzees at 6–8 million years. Comparative genomics from complete genome sequencing to chromosome painting provide a scenario for the origin of the human genome. Starting form the ancestral mammalian karyotype, we can determine the major steps over the last 90 million years leading to the formation of each human chromosome. Despite considerable technical problems, studies of ancient DNA now provide a direct genetic witness of human evolution and add a temporal dimension to reconstructions of our evolutionary history and phylogeny. Ancient DNA has shown that Neanderthals probably did not interbreed with anatomically modern humans and did not make a significant contribution to the gene pool of our species. Ancient DNA has also contributed to the studies of the colonization of the Americas and the Pacific Island, and the domestication of plants and animals. Understanding the genetic basis of the physical and behavioral traits that distinguish humans from other primates presents one of the great future challenges of science.